Thu Dec 23, 2004
Listen in RealAudio
What happens to water when it gets below 32 degrees Fahrenheit? Duh, it freezes,
right? Well, not always, and that can lead to an interesting structure known as "rime
ice." Hi, I’m Bryan Yeaton, and this is The Weather Notebook.
Water isn’t required to freeze at 32 degrees, or zero Celsius. Call it more of a strong
suggestion. In order to freeze, water needs a little help—a condensation nucleus.
These nuclei help cloud droplets to form by giving the water molecules something to
grab onto. They also provide an impetus for freezing. So, without this catalyst, the
droplets—which are very tiny, about a million in one raindrop—can stay in liquid form
even down to minus 40 degrees before they freeze spontaneously. Meteorologists call
these "super-cooled" droplets.
In air colder than 32 degrees, some cloud droplets will be frozen and some will be
super-cooled. When they encounter an object, the frozen ones will simply bounce off
and continue their trek the wind. But the super-cooled droplets now have something to
freeze to, and boom! You have a tiny piece of ice.
Now, in a cloud, billions of these drops can pile into each other every second, freezing
in fractal formations that grow INTO the wind. These beautiful patterns can easily be
crumbled in the hand, and are a brighter white than the more blue, solid form. The
reason for this is that a great deal of air is trapped with the structure as it grows,
sometimes more than a foot an hour.
Forms of the word "rime" can be found in Ole and Middle English, as well as French
and Old Norse. To see some great pictures of rime ice, taken right at our office atop
Mount Washington, go to our website, www.weathernotebook.org. Our show is
produced with funding from Subaru of America.
Rime ice on Mount Washington Observatory buildings: